The exact molecular structures of some highly toxic polyhalogenated aromatic hydrocarbons have been obtained by X-ray crystallographic measurements or estimated based on postulated structures. The requirements of molecular symmetry and size as determined by the number, kind, and positions of halogens, planarity, interatomic distances and overall stereoelectronics suggest that a specific biological receptor may be involved which could account for the common toxic pattern. The guinea pig will continue to be used as a screening animal model in providing direction for further biological study of related compounds. A possible relationship of this toxicity and carcinogenicity and cocarcinogenicity in ubiquitous polynuclear aromatic hydrocarbons is emerging.